The main objective of this study was to determine the relationship of financial leverage on financial efficacy of the Kenya manufacturing companies listed in Nairobi securities exchange over a period of seven (7) years (2011 – 2017). Specifically, the study sought to examine the effect of debt financing on financial efficacy of listed manufacturing companies in Kenya and to investigate the effect of firm’s size as a moderating variable on the relationship between financial leverage and Financial efficacy of manufacturing firms in Kenya. The study was based on Modigliani and Miller Proposition I and II, the trade-off theory, pecking order theory and the agency theory. The research adopted a descriptive research design. The target population for the study was staff members of the listed manufacturing firms in Kenya. The target constituted respondents from, accounting department, finance department, Auditing and Assurance Department and Monitoring and Evaluation Department of listed manufacturing firms in Nairobi securities exchange in Kenya. A sample of 106 respondents was selected by use of stratified random sampling. Data was collected through a structured questionnaire. Both descriptive and inferential statistics were used to analyze the data. Data presentation was done by the use of charts and tables for ease of understanding and interpretation. Both primary and secondary data were used. Pilot study was conducted by the researcher taking some questionnaires to the listed manufacturing firms head offices in Kenya. From this pilot study, the researcher was able to detect questions that need editing and those that will be ambiguous. The study used Cronbach (Alpha – α) model to test the internal consistency with the alpha coefficient of above 0.7 being considered reliable. To establish the validity of the research, instrument the research pursued the opinions of experts in the survey of study especially the researcher’s supervisors. Quantitative and qualitative data that were collected using questionnaires and the questionnaires were inspected for errors and gaps before issuing to the respondents. The findings revealed that debt financing, equity financing and retained earning positively and significantly influenced financial efficiency among the listed manufacturing firms. As well, firm size positively moderated the relationship between financial leverage (debt financing, equity financing and retained earnings) and financial efficacy of the manufacturing firms. The study recommended for firms to put in place modalities for determining and evaluating the optimal sources of business finance. Once this is done, the purpose of borrowing funds needs to be defined at the onset and a scoring system should be in place for evaluating the viability of the sources of debt for firms. Also, emphasis needs to be on ensuring that equity financing is in line with the objectives of the firm. Finally, there is need for firms to only retain earning on occasions when there are investment projects that are likely to enhance financial efficacy.